What Is Ghosting On Timber Floors? An Expert Explains

In terms of wooden floors, ‘ghosting’ traditionally refers to the appearance of lightly coloured boot prints that appear in a floor finish, normally a considerable time after the floor has been coated. Homeowners will often begin to see unusual lighter patches in the coating, and over time these become more obviously a boot print. Confusion then sets in with the homeowner wondering how a boot print that doesn’t match theirs could suddenly appear in their home?

To add to the confusion, ghosting on wooden floors is also not just confined to boot prints and it doesn’t always appear on horizontal surfaces. It can relate to handprints, bare feet and cloudy smudges with no predetermined shape. These marks differ from contact staining (often called ‘tattooing’) that refers to imprints that occur when people walk on a wet coating.

What do we know about ghosting?

Ghosting on timber floors is a bit of an unusual phenomenon due to its intermittent nature, and the fact that it affects relatively few floors. In fact, the problem is still not fully understood and hasn’t been fully solved. It has been investigated to some degree, however this has often left even more questions that don’t fit the previous explanations!

What we do know is that reports of ghosting only started in the late 90’s, around the time that water-based polyurethanes and fast-dry solvent sealers (vinyl type) began to enter the market. These sealers

enhanced the productivity of a job because they allowed the first of the final coats, for both solvent and water-based finishes, to be applied on the same day. This enhanced productivity meant a quicker job was achieved, which was obviously a benefit for both contractor and client!

What is ghosting caused by?

It is thought that some ghosting is caused by the absorption of oils from bare feet and (more often) footwear

into partially cured coatings that contain residual solvents. These residual solvents rapidly dissolve the oils from certain footwear and even bare skin, and these oils remain dormant within the cured coatings. That is, until they are gradually broken down by UV light in a ‘radiation-induced photo-oxidative degradation’. Throughout this breakdown, the oils develop an amorphous structure that’s visible under a microscope at 200 times the magnification.

What’s an amorphous structure? Solids are arranged in two basic ways – they can form a regularly repeating three-dimensional structure known as a crystal lattice (thus producing a crystalline solid), or they can aggregate in no particular order, in which case they form an amorphous solid. This amorphous structure is typical of the photo-oxidation breakdown that happens in polymers, and oils are polymers. The result is a gradually appearing ‘ghost’ reproduction of the object that was in contact with the floor.

What’s needed for ghosting to occur?

In many of the case studies done it was noted that ghosting on timber floors tended to appear where boots had elastomer or urethane thermoplastic soles as opposed to nitrile or neoprene vulcanised rubber soles. The majority of prints were found within two metres of walls and the weather conditions at the time were warm and humid. Slower drying, water and solvent-based sealers were also used and it’s worth noting that humid conditions actually retard solvent evaporation.

Thermoplastic soles are not solvent resistant and show tackiness when they are rubbed with solvent. The two solvents used are toluene, which is used in some solvent-based sealers, and glycol ether that is often used in water-based sealers. All of these factors form an equation that incorporates certain weather conditions and a particular boot and sealer type, which can all contribute to ghosting.

The final component needed is UV light, which, with time, turns the invisible footwear polymers that have been dissolved in the coating into a visible milky-coloured compound of the shape that made it. in terms of bare skin or bare feet, for example, it is generally thought that the oils present are similarly dissolved in the coating and over time also develop a milky colour under UV light. These processes have also been replicated in a laboratory environment, adding more proof to the hypothesis.

What other factors influence ghosting?

Where ghosting on timber floors appears adjacent to an ensuite or near a patio exit onto a paved area, it is often questioned whether the tiled, marbled or paved area has had an influence. However, mineral contaminants such as plaster and similar compounds are not considered to be factors in producing ghosting.

Others have questioned whether perspiration has any impact. Perspiration is known to produce small, milky circular marks both on and between water-based coatings, however, this is not the case with solvent-based coatings. Therefore, it’s unlikely to be the cause of the ghosting seen in both solvent and water-based finished floors.

What can be done about ghosting?

Ghosting on wooden floors can essentially be reduced by wearing ghosting-solvent and oil-resistant footwear. It’s also essential that the sealer coat is given ample time to fully dry, especially in high humidity or in cooler weather. This is particularly the case with fast-dry sealers. The above not only outlines ‘good practice’ when it comes to coating floors, but also highlights the fact that it is essential that homeowners and other contractors don’t walk on flooring until the flooring contractor permits it. Basically, the floor is a ‘no go’ zone except to the flooring contractor!

There is some anecdotal evidence to suggest that although it’s often considered to be present between the sealer and the final coats, ghosting may be corrected by sanding back to the bare timber. This has been known to work, however not every time, as in some instances after re-sanding and coating, the original ghosting has reappeared. This has led to some people suggesting that the timber itself is affected, and due to ghosted boot prints being present in some stair risers, some credence can be given to this viewpoint.

A final word about ghosting

It’s worth pointing out that although ghosting on timber floors is a problem that needs to be addressed, it is not common. There are also a variety of factors involved, and ghosting can occur in a number of different ways. Some of these are well understood, and others not so well! Also, some factors can probably be discounted as being associated with ghosting on wooden floors, however, there are still some aspects of the whole issue that need to be investigated further.

However, most will agree that ghosting can be induced into the raw timber, can appear between coatings and can also appear some years after a coating has been applied. There is some level of risk in attempting to fix it as it may reappear, but not in all instances. Therefore, blame cannot really be apportioned. The best result is for a resolution agreed to by all parties in terms of sharing the cost of repairs and/or agreeing to what is the best course of action.